1. Field of the Invention
The present invention relates to a spark plug used for an internal-combustion engine.
2. Description of the Related Art
A spark plug for internal-combustion engines is mounted on an internal-combustion engine, and is used for igniting an air-fuel mixture in a combustion chamber. Generally, a spark plug is comprised of an insulator having an axial bore, a center electrode inserted in a front end of the axial bore, a terminal electrode inserted in a rear end of the axial bore, a metal shell provided in an outer circumference of the insulator and a ground electrode provided on a front end portion of the metal shell, and forming a spark discharge gap with the center electrode.
Along with an operation of an internal-combustion engine, conductive carbon is accumulated on a surface of the insulator. Especially, when a front end of the insulator positioned in a surrounding of the center electrode is covered by carbon, the current applied to the center electrode is transmitted and leaks to the metal shell through the carbon adhered to the front end of the insulator. As a result, a normal spark discharge is less likely conducted (i.e., misfire). Therefore, it is disclosed that the front end portion of the ground electrode is disposed so as to face a side surface of the center electrode, thereby generating the spark discharge between two electrodes through the front end face of the insulator (e.g., refer to Patent Document 1). The conventional art can burn off the carbon adhering to the front end of the insulator at the time of the spark discharge, and excellent anti-fouling characteristics are materialized.
In recent years, improvement in ignitability has been demanded in order to improve fuel consumption and emission reduction. When the above-mentioned conventional art is adopted, a spark discharge is conducted in a position away from the center of a combustion chamber. Thus, there is a possibility that the conventional art may have insufficient ignitability.
Therefore, in order to improve the ignitability, a side portion of a ground electrode is disposed so as to face the front end portion of the center electrode, as well as a noble metal tip having a relatively small diameter is provided on a face opposed to the center electrode or the ground electrode (e.g., refer to Patent Document 2). According to this conventional art, the sparks can be discharged near the center of the combustion chamber. Further, the conventional art can prevent the heat of sparks (flame kernel) from being conducted through the center electrode or the ground electrode.
However, in light of environmental regulations, a direct-injection engine has been used recently in order to facilitate an energy saving and to control discharge of unburnt gas or the like. However, since the direct-injection engine injects fuel into or near a spark discharge gap, carbon tends to be accumulated on a front end portion of an insulator. When the above-mentioned art is adopted, it can hardly burn off the carbon adhering to the front end of the insulator although it can improve ignitability. As a result, anti-fouling characteristics tend to be insufficient which may cause a misfire.
On the other hand, it is disclosed that a position of spark discharge is made near the center of a combustion chamber by disposing a front edge of the ground electrode to face a front edge of the center electrode so that ignitability may improve. Furthermore, the carbon accumulated to the surface of the insulator can be burnt off (e.g., refer to Patent Document 3).
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. H10-50455
[Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 2005-108795
[Patent Document 3] Japanese Patent Application Laid-Open (kokai) No. 2004-55142
3. Problems to be Solved by the Invention
However, in various spark plugs, a position of a front end portion of an insulator in relation to a front end portion of a center electrode vary (e.g., an outer diameter of the front end portion of the center electrode is equal to or differ from an inner diameter of the front end portion of the insulator, or the front end portion of the insulator is close to or away from the front end portion of the center electrode). That is, the conventional art stated in the above-mentioned Patent Document 3, which specified a physical relationship between the center electrode and the ground electrode, is not fully examined about improvement in anti-fouling characteristics. It may not realize the improvement in anti-fouling characteristics in various spark plugs. According to this conventional art, spark discharge is conducted between the insulator and the ground electrode even though the insulator is not fouled. As a result, improvement in ignitability may not fully be demonstrated.
The present invention has been accomplished in view of the foregoing, and an object of the present invention is to provide a spark plug for internal-combustion engines which has an excellent ignitability and a sufficient anti-fouling characteristics regardless of a position of an insulator in relation to a center electrode.